High speed image sensors have been widely used in many applications in different fields including the automotive field, the machine vision field, and the field of professional video photography. The development of high speed image sensors is further driven by the consumer market's continued demand for high speed slow motion video and normal high-definition (HD) video that have a reduced rolling shutter effect.
In conventional complementary metal-oxide semiconductor (“CMOS”) pixel cell, image charge is transferred from a photosensitive device (e.g., a photodiode) and is converted to a voltage signal inside the pixel cell on a floating diffusion node. The image charge can be readout from the pixel cell into readout circuitry and then processed. In conventional CMOS image sensors, the readout circuit includes an analog-to-digital converter (ADC). ADCs inherently suffer from non-linearity errors due to the particular architecture of each ADC. The non-linearity errors, which include the integral non-linearity (INL) and the differential non-linearity (DNL), cause the output of the ADC to deviate from the ideal output. For instance, the ideal output may be a linear function of the input.
Since these non-linearity errors are inherent to the ADCs, it is not possible to remove the effects of these errors by calibration. The negative effects of the non-linearity errors on the image sensor include a reduction of the dynamic range of the image charge (e.g., input signal) that may be processed by the readout circuitry's ADC as well as a reduction of the effective resolution of the ADC.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.